Shrinkable permeable texture film

ABSTRACT

POLYETHYLENE FILMS, HIGHLY PERMEABLE TO OXYGEN, ARE MADE BY SINTERING GRAINS OF HIGH-DENSITY POLYETHYLENE, IRRADIATING THE RESULTING SHEET TO ABOUT 3 MEGARADS, MELTING OUT THE SINTERED STRUCTURE, AND IRRADIATING A SECOND TIME.   WHEN THE SHEET IS STRETCHED, OXYGEN PERMEABILITY IS INCREASED AS MUCH AS 10 TIMES.

y 1973 R. s. GREGORIAN ETAL 3,736,218

SHRINKABLE PERMEABLE TEXTURE FILM Original Filed Jan. 10, 1969ESSENTIALLY H/Gh DENSITY POLYE T HYLE/VE OXYGEN PE RMEABLE TEXTURED F/LMUnited States Patent SHRINKABLE PERMEABLE TEXTURE FILM Razmic S.Gregorian, Arken, S.C., Charles C. Kirk, Laurel, Md., and James A. Cote,Arlington Heights, Ill., assignors to W. R. Grace & Co., New York, N.Y.Original application Jan. 10, 1969, Ser. No. 790,422, now Patent No.3,576,927, dated Apr. 27, 1971. Divided and this application Oct. 19,1970, Ser. No. 82,135

Int. Cl. B32]: 27/06, 27/16, 33/00 US. Cl. 161-164 1 Claim ABSTRACT OFTHE DISCLOSURE Polyethylene films, highly permeable to oxygen, are madeby sintering grains of high-density polyethylene, irradiating theresulting sheet to about 3 megarads, melting out the sintered structure,and irradiating a second time. When the sheet is stretched, oxygenpermeability is increased as much as times.

This is a division of application Ser. No. 790,422, filed Jan. 10, 1969,now US. Pat. No. 3,576,927 issued Apr. 27, 1971.

This invention relates to the production of films which are permeable tooxygen. Such films have various specialized uses, but they areespecially advantageous in the preservation of beef kidneys in frozenstorage where oxygen is necessary to preserve the color of the meat. Atthe same time, an oxygen-permeable film protects the meat from handlingwhile on the display counters, and prevents bacteria and mold sporesfrom coming in contact with the meat. It also, to a very materialdegree, cuts down the transfer of water vapor.

We have discovered that if a film of high density polyethylene isprepared in the manner which this specification will set forth, theoxygen permeability of the film jumps by at least one order of magnitudeand even more. Interestingly, this effect can only be secured if thedensity of the resin is high. Low-density material treated in the samemanner shows little or no increase in oxygen permeability.

Our invention takes advantage of the fact that if grains of high-densitypolyethylene are sintered under heat and pressure so that a coherentsheet is formed, and if that sheet thereafter is subjected to electronbombardment to a mild dosage, the sintered structure of the sheet willpersist through the further processing steps which are necessar S heetswhich exhibit this persistent structure have markedly increasedpermeability to oxygen.

The effects which we describe are at a maximum if all of thepolyethylene has a density of 0.93 or higher. If low density(high-pressure polyethylene) appears in the highdensity material, theoxygen permeability of the resulting sheet becomes less marked and isinsignficant if as much as 10 of low-density polyethylene appears in themixture.

The single figure of the drawing shows the persistent structure of thefinished sheet.

EXAMPLE 1 Granules of high-density 0.95 polyethylene, melt index 5.0,were pressed between polished sheets to produce a sintered structure ofan inch thick. The sheet was then exposed to the beam of a High-VoltageEngineering electron accelerator and given a dose of 3 megarads.Thereafter the sheet was fused under pressure and at 350 F. until thesintered structure appeared to be melted out. The sheets were thenallowed to cool, and thereafter were exposed to a further electronbombardment to a dosage of '3 megarads. Following the secondirradiation, the sheet "ice was stretched in both directions to 2 timesthe length of the sintered sheet. During biaxial stretching thecondition of the sheet changes materially and a visual texture appears.The oxygen permeability of the resulting sheet was 26.22 10 cc.mil/ATM--M Day.

Irradiating the high-density polyethylene to a total dosage of 6megarads in two successive steps appears to have the following effects:the first irradiation crosslinks the sintered particles to a degreewhich imparts a lockedin sintered structure memory which persists eventhough the sheets are fused in the press to replace the sinteredstructure with a smooth, coherent polyethylene sheet. The subsequentirradiation develops sufficient additional crosslinks to allow the sheetto be stretched biaxially. It is during this biaxial stretchingoperation that the sintered structure of the sheet is somewhatreproduced and remains visually evident in the oxygen-permeable texturedfilm.

EXAMPLE 2 (Control example) Grains of 0.95 high-density polyethylene,Grex melt index 5, were placed between polished sheets and pressed at350 F. to produce a sintered sheet of an inch thick. The sheets werethen fused at 350 F. until the sintered structure disappeared. Aftercooling, the sheet was stretched to 2 /2 times its original dimensionboth lengthwise and transversely of the sheet. The oxygen permeabilityof Example 2 was determined and found to be 2.1 X 10 cc. mil/ATM-M Day.

EXAMPLE 3 A blend of 5% by weight of low-density polyethylene, meltindex 12 (Microthene 704), and of 0.95 highdensity polyethylenecopolymer, melt index 5.0 (Grex 50-050 C.) made by melting the twopolyethylenes, blending the liquid mixture, and granulating the chilledblend was sintered by heating under pressure, and the followingtemperature conditions: top platen temperature, 235-250 F.; bottomplaten temperature, 250-270 F.; pressing time, 0.25-0.35 minute. Thesintered sheets were allowed to cool and then were irradiated as abovedescribed to a dosage of 3 megarads. Following irradiation, the sheetwas placed under pressure and raised to 350 F. to melt out the sinteredstructure. Following cooling, the sheet was subjected to a secondelectron bombardment of 3 megarad dosage. The oxygen permeability of thesheet was subsequently determined and found to be 3.27 10 cc. mil/ATM-MDay.

EXAMPLE 4 A control sheet made from the same blend of lowandhigh-density polyethylene utilized in Example 3 was made by followingall of the steps recited in Example 3 except the sheet received noirradiation either in the first or sec- 0nd step. Theoxygen-permeability of this sheet was tested after the sheet had cooledand was found to be 192x10 cc. mil/ATMM Day. The foregoing tests showhow sheets can be produced which can cover a very considerable range ofoxygen permeability. Very small amounts of low-density polyethyleneadded to the blend drop the oxygen permeability as contrast betweenExample 1 and Example 3 shows.

What is claimed is:

1. A film possessing materially increased oxygen permeability consistingof a crosslinked oriented ethylene polymer having a density of at least0.93 characterized by the presence of a visually evident sinteredtexture in the film, said film having an oxygen permeability of from3.27 10 cc. mil/ATM-M Day to 26.22 10 cc. mil/ ATMM Day.

(References on following page) References Cited UNITED STATES PATENTSBernstein 264-22 Rainer et a1. 264-22 Waddell 264-294 Bennig et a1.264-95 Statton 264-126 Rainer et a1 264-22 4 2,904,480 9/1959 Rainer eta1. 264-22 GEORGE F. LESMES, Primary Examiner G. W. MOXON II, AssistantExaminer US. Cl. X.R.

